Toshiaki Kodama

Ability of the V3 Loop of Simian Immunodeficiency Virus To Serve as a Target for Antibody-Mediated Neutralization: Correlation of Neutralization Sensitivity, Growth in Macrophages, and Decreased Dependence on CD4

ABSTRACT To better define the effects of sequence variation and tropism on the ability of the simian immunodeficiency virus SIVmac V3 loop to act as a target of antibody-mediated neutralization, a series of experiments were performed. Three SIV strains, SIVmac239, SIVmac316, and SIVmac155/T3, each with defined differences in envsequence and tropism, were used to construct a panel of viruses chimeric for a portion of envelope that includes the V2 and V3 regions. Peptides with sequences corresponding to the V3 loops of the parental viruses were used to immunize rabbits. The polyclonal rabbit antibodies and plasma from SIVmac239-infected animals were then used to assess the neutralization sensitivity of the parental and chimeric viruses. One of the parental viruses, SIVmac316, which is able to replicate to high titer in alveolar macrophages and can infect cells in a CD4-independent fashion, was highly sensitive to neutralization by plasma from SIVmac-infected rhesus macaques, with average 50% neutralization titers of 1:20,480; this same strain was also sensitive to neutralization by the anti-V3 loop peptide sera. Other parental and chimeric viruses were less sensitive to neutralization with this same panel of antibodies, but as seen with SIVmac316, those viruses that were able to productively replicate in alveolar macrophages were more sensitive to antibody-mediated neutralization. To further define the amino acids involved in increased sensitivity to neutralization, a panel of viruses was constructed by changing envelope residues in SIVmac316 to the corresponding SIVmac239 amino acids. The increased neutralization sensitivity observed for SIVmac316 was mapped principally to three amino acid changes spread throughout gp120. In addition, the increased sensitivity to neutralization by V3-directed antibodies correlated with the ability of the various viruses to replicate to high levels in alveolar macrophage cultures and a CD4-negative cell line, BC7/CCR5. These results demonstrate that the V3 loop of SIVmac Env can act as an efficient target of neutralizing antibodies in a fashion that is highly dependent on sequence context. In addition, these studies suggest a correlation between decreased dependence on CD4 and increased sensitivity to antibody-mediated neutralization.

Characterization of Monoclonal Antibodies that Distinguish Simian Immunodeficiency Virus Isolates from each Other and from Human Immunodeficiency Virus Types 1 and 2

Two monoclonal antibodies (MAbs) against p27 and one against p17 of simian immunodeficiency virus (SIV) from rhesus macaques were produced and characterized by reacting with disrupted, viral antigens on immunoblots. Human immunodeficiency virus type 1 (HIV-1), HIV-2 and SIV isolates from sooty mangabey, stump-tailed macaque, rhesus macaque and African green monkey (SIVSM, SIVStM, SIVMAC and SIVAGM) were used for comparative analysis. The p27 monoclonal antibodies HE3 and FA2 reacted with SIVMAC and SIVSM, but not with HIV-1, HIV-2, SIVStM and SIVAGM. The p17 monoclonal antibodies reacted with SIVMAC and SIVStM, but not HIV-1, HIV-2, SIVSM and SIVAGM. The differential reactivity of these monoclonal antibodies indicated that common conserved antigenic epitopes are shared between SIVMAC and SIVSM with respect to p27 MAbs and between SIVMAC and SIVStM with respect to p17. Since these MAbs reacted differently with the SIV isolates, they are useful reagents for comparative pathogenesis studies for differentiating SIV isolates.

Comparison of Plasma Viremia and Antibody Responses in Macaques Inoculated with Envelope Variants of Single-Cycle Simian Immunodeficiency Virus Differing in Infectivity and Cellular Tropism

ABSTRACT Molecular differences in the envelope glycoproteins of human immunodeficiency virus type 1 and simian immunodeficiency virus (SIV) determine virus infectivity and cellular tropism. To examine how these properties contribute to productive infection in vivo, rhesus macaques were inoculated with strains of single-cycle SIV (scSIV) engineered to express three different envelope glycoproteins with full-length (TMopen) or truncated (TMstop) cytoplasmic tails. The 239 envelope uses CCR5 for infection of memory CD4+ T cells, the 316 envelope also uses CCR5 but has enhanced infectivity for primary macrophages, and the 155T3 envelope uses CXCR4 for infection of both naive and memory CD4+ T cells. Separate groups of six rhesus macaques were inoculated intravenously with mixtures of TMopen and TMstop scSIVmac239, scSIVmac316, and scSIVmac155T3. A multiplex real-time PCR assay specific for unique sequence tags engineered into each virus was then used to measure viral loads for each strain independently. Viral loads in plasma peaked on day 4 for each strain and were resolved below the threshold of detection within 4 to 10 weeks. Truncation of the envelope cytoplasmic tail significantly increased the peak of viremia for all three envelope variants and the titer of SIV-specific antibody responses. Although peak viremias were similar for both R5- and X4-tropic viruses, clearance of scSIVmac155T3 TMstop was significantly delayed relative to the other strains, possibly reflecting the infection of a CXCR4+ cell population that is less susceptible to the cytopathic effects of virus infection. These studies reveal differences in the peaks and durations of a single round of productive infection that reflect envelope-specific differences in infectivity, chemokine receptor specificity, and cellular tropism.